Navigation systems for vehicles that are equipped with a function for displaying a roadmap in the vicinity of the vehicle position, a function for accurately detecting the vehicle position through map matching, a function for calculating a recommended route from a point of departure to a destination, a function for implementing route guidance based upon a calculated recommended route and the like exist in the known art. In these navigation systems for vehicles in the prior art, roadmap display data, map matching data, route search data, route guidance data and the like are stored in a map database apparatus such as a single CD-ROM in order to maintain compatibility with existing software and to improve processing speed.
In the case of the map display data, they are normally managed by dividing a given map area into a plurality of portions in order to display the map on a monitor or the like of the navigation system. These divided units are referred to as meshes. FIG. 15 illustrates a map area divided into 25 portions with each mesh assigned with codes A, B, C . . . X and Y. If the map is currently displayed using the data corresponding to the mesh A, the map display data corresponding to the meshes surrounding the mesh A are likely to be needed as the vehicle advances or through a scroll operation performed by the user. In systems in the prior art, management is implemented by providing each mesh with data addresses of the meshes surrounding it and in the case of a CD-ROM, by providing each mesh with sector addresses on the CD-ROM of the meshes surrounding it. For instance, the mesh A holds addresses corresponding to the 8 meshes; B, C, D, E, F, G, H and I that surround it, whereas the mesh B holds addresses corresponding to the 8 meshes; K, L, C, D, A, H, I and J that surround it. Since four bytes are required to express an address in the CD-ROM, a 32-byte area is required at each mesh in order to hold the addresses of the surrounding meshes. The same principle applies to the route search data and the route guidance data.
The great data quantity required in a map database apparatus that stores map display data and the like in order to implement the processing through which data corresponding to surrounding meshes are obtained from a given mesh in this manner, results in the necessity for securing a very large data storage area.